The present invention generally relates to phase change memory devices, more particularly, to phase change memory device capable of increasing a write current flowing in a phase change resistor for improving the cells driving capacity.
Nonvolatile memory devices that include magnetic memory devices and phase change memory (PCM) devices have data processing speeds similar to those of volatile Random Access Memory (RAM) devices. Furthermore nonvolatile memory devices enjoy the advantage associated with conserving data even after the power is turned off.
FIGS. 1a and 1b are diagrams illustrating a conventional phase change resistor (PCR) 4.
The PCR 4 comprises a phase change material (PCM) 2 inserted between a top electrode 1 and a bottom electrode 3. When an electrical signal having a voltage and a current is transmitted through the PCM 2, an elevated temperature can be generated in the PCM 2 so that the electric conductive state of the PCR 4 can be controlled or changed depending on whether or not the heated PCM 2 can be slowly cooled as a crystalline lattice structure or rapidly cooled as an amorphous lattice structure. That is the resistance of the crystalline lattice of the PCM 2 exhibits a lower resistance than the resistance of the amorphous lattice of the PCM 2.
One PCM 2 of interest includes AgLnSbTe. The PCM 2 includes chalcogenide having chalcogen elements (S, Se, Te) as a main ingredient. Another PCM 2 of interest includes the germanium antimonic tellurium (Ge2Sb2Te5).
FIGS. 2a and 2b are diagrams illustrating a principle of the conventional PCR 4.
As shown in FIG. 2a, the PCM 2 can be crystallized when relatively low currents of less than a threshold pass through the PCM R. As a result, the PCM 2 can be crystallized to exhibit a low resistant material.
As shown in FIG. 2b, the PCM 2 has a temperature of a more than a melting point when a high current of more than a threshold passes through the PCR 4. As a result, the PCM 2 can become an amorphous lattice that exhibits a relatively high resistance.
In this way, the PCR 4 can be configured to store nonvolatile data corresponding to the two resistance states. For instance, a logical data state of “1” can be assigned to correspond to the PCR 4 when at a low resistance state. Likewise, a logical data state of “0” can be assigned to correspond to the PCR 4 when at a high resistance state. In this way, the logic states of the two data can be stored.
FIG. 3 is a diagram illustrating a write operation of a conventional phase change resistant cell.
Heat is generated when a current flows between the top electrode 1 and the bottom electrode 3 of the PCR 4 for a given amount of time. As a result, a state of the PCM 2 can be changed to be either crystalline or amorphous depending upon what temperature was applied between the top electrode 1 and the bottom electrode 3.
When a low current flows for a given time, the PCM can become crystalline during a low temperature heating state so that the PCR 4 can be set to a low resistive set state. On the other hand, when a high current flows for a given amount of time, the PCM can become amorphous due to the generated high temperature heating state so that the PCR 4 can be set to a high resistive reset state. A difference between two phases is representative of an electric resistance change.
A low voltage can be applied to the PCR 4 for a relatively long time period in order to write the set state in a write mode. On the other hand, a high voltage can be applied to the PCR 4 for a relatively short time period in order to write the reset state in the write mode.